1. Field of the Invention
The present invention relates to a method of increasing the useful length of life of heating elements when heat treating electronic ceramics. Such ceramics may be ferrites (magnetic ceramics) and titanates (e.g., BaTiO3).
More specifically, the invention pertains to elements of the types molybdenum silicide and molybdenum tungsten silicide, including different alloys of these basic materials. Such elements are manufactured by applicants in a number of designs.
2. Description of the Related Art
Electronic ceramics are at present used in a number of applications in, e.g., optronics, mobile telephony and vehicle electronics. Electronic ceramics that have a perovskite structure have dielectric, piezoelectric, and ferroelectric properties, among other things. Examples of such materials include BaTiO3 and Pb(Zr, Ti)O3. Components that are typically manufactured from such materials are resonators, filters, and capacitors. The capacitors are produced in the form of so-called multi-layer capacitors (MLCC) for example. The ceramics are often brought into contact with a metallic conductor, such as nickel, in the heat treatment stage.
When such heating elements are used in the manufacture of so-called electronic ceramics, the elements are often placed on a supportive surface and heated respectively from the floor and ceiling of continuous sintering furnaces. A long preferred design of heating elements in such furnaces, for instance furnaces of the so-called Riedhammer type for so-called ferrite sintering furnaces, are so-called 4-arm meander elements measuring {fraction (6/12)} mm on the heating zone and connection part respectively.
Such elements rest typically on a supportive gravel layer of aluminum-silicate particles, which, in turn, rests on a brickwork of purely aluminum oxide. The aluminum silicate, which is compatible (does not react chemically) with heating elements of MoSi2 up to a temperature of 1600° C., is of the type sillimannite and mullite, respectively. Sillimannite and mullite can be used for a long period of time before a reaction takes place with the SiO2-layer that develops on the surface of MoSi2 elements. This is because the aluminum silicate reacts more slowly with the SiO2-layer than the aluminum oxide brick. This reaction causes aluminum to be alloyed in the SiO2-layer, therewith weakening the ability of the layer to protect the element material, resulting in a shorter life length of the element.
Subsequent to this reaction with the surface layer a reaction also takes place in the bulk material of the heating element, resulting in further corrosion and weakening.
In the absence of said particle layer, or gravel bed, a quicker reaction will take place by virtue of the SiO2 on the surface of the element being in direct contact with the Al2O3-brick.
Many electronic ceramics are heat treated at furnace temperatures ranging from 1200 to 1500° C. and higher. In addition to nitrogen gas, the atmosphere also typically contains about 5% hydrogen gas that has a varying dew point. For instance, the dew point may be +20° C.
It has been noted that in many cases the useful life length of said elements falls significantly beneath the expected 3-5 year span, and in some cases only extends to a few months. The cause of local life length problems is the aggressive hydrogen gas that reacts with the gravel bed such as to form a smelt that contains chiefly aluminum silicate. The heating element sinks down in the smelt, and an increase in temperature takes place with accelerated corrosion and temperature increase of the elements as a result. Moreover, the gravel bed sinters firmly into the aluminum oxide brick. The problem can also occur in atmospheres that do not contain hydrogen gas, wherein the element buries down into the particle layer as a result of temperature changes and therewith dimensional changes.
The gravel bed, or particle layer, also makes manufacture, transportation, and furnace servicing more difficult to carry out.
It is therefore desired to eliminate the gravel bed from the process. The present invention fulfils this desire.